We have been interested in defining the major mechanisms of simultaneous resistance of cancer cells to multiple chemotherapeutic agents. One major mechanism is expression of an energy-dependent efflux pump, termed P-glycoprotein (P-gp), or the multidrug transporter, encoded in humans by the MDR1 gene. The sequence of the MDR1 cDNA led to a model of the transporter as a pump with 12 transmembrane domains and 2 ATP sites; determination of the domains of P-gp responsible for substrate binding and coupling of ATPase activity to substrate transport are the major goals of our work. Model systems based on stable expression or transient expression of mutated P-gps have been developed to assay functional effects of these mutations on drug binding, drug-dependent ATPase, drug resistance and drug transport. The creation of expression vectors able to confer multidrug resistance and the demonstration that expression of the MDR1 gene in the bone marrow of mice leads to resistance to anti-cancer drugs have enabled the development of vectors for gene therapy of cancer and other genetic diseases in which P-gp serves as a dominant selectable marker; improvements on these vectors leading to trials in animal models and eventually in patients are another important goal of our research. We have also begun to explore the mechanism of multidrug resistance resulting from selection in cisplatin of hepatoma cells and the yeast Saccharomyces cerevisiae. Cisplatin-resistant hepatoma cells accumulate reduced amounts of cisplatin by a mechanism as yet to be determined, and at least two yeast genes associated with cisplatin resistance have been isolated.